The present invention relates to a volume control circuit and, mpre particularly, to a volume control circuit which is applicable to a portable, vehicle-mounted telephone and to other equipment which are usable in any desired location.
A portable telephone, for example, is designed to be usable inside and outside of a motor vehicle, as desired. Usually, it has a volume control circuit to facilitate communications at an optimal sound level with no regard to the environment, i.e. both in the inside and the outside of a motor vehicle. Specifically, the sound level is raised while the telephone is used at a fixed position in a vehicle compartment (hereinafter referred to an "onboard condition"), which suffers from much noise, and is lowered while it is carried by a person outside of the vehicle compartment (hereinafter referred to as a "portable condition").
A drawback with a prior art volume control circuit is that the sound level has to be adjusted every time the condition under which the telephone is used is changed, as mentioned above. This level adjustment adds to the labor required for communication and, therefore, makes the telephone inconvenient to use.